This episode stands out first of all because it takes Gary to yet another time period, leading to new time jokes, and because it succeeds in doing something that I feel the show hasn't been good at so far: it humanizes Yvonne.
The premise of Goodnight Sweetheart is that Gary finds it difficult to choose between his two wives, who live in different times. As the largest part of the show takes place in the 1940s Phoebe gets most of the attention. I have always found it difficult to sympathise with Yvonne, who is shown as a cold, selfish, yuppie. At the start of this episode she has become slightly more likeable, as she has sold her company and is even willing to lend Ron some money.
But she comes to life, as it were, when Gary accidentally winds up in Victorian England and meets her ancestor, a music hall singer. This incarnation of Yvonne is innocent and likeable, possibly reminding Gary of the Yvonne he first met. The way he looks at her makes his marriage to the 1999 Yvonne believable.
The new time period gives the author a chance to play around with known characters. Reg's ancestor not only proves to be intelligent but also in possession of a sense of humour. Gary's attempts at telling jokes fail because he realises that every reference he makes leaves the Victorian audience clueless. The time travelling gimmick adds a new theory to the fate of Jack The Ripper.